|M.Sc Student||Leb Adam Dan|
|Subject||Increasing the Energy Efficiency of Seawater Desalination|
Through Osmotic Energy Recovery
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Guy Ramon|
|Full Thesis text|
Sea water reverse osmosis (SWRO) is a large energy consumer due to the pressure required to overcome the osmotic pressure, and as long as the main source for electricity are fossil fuels, it is polluting and unsustainable. Salinity-gradient energy, which is the energy available from mixing two aqueous solutions of different salinities, is a high-potential sustainable power source that can be a part of the mitigation. It can be used as a direct energy source, or to enhance the energy efficiency of desalination plant. The total estimated global potential for salinity gradient power (SGP) production is 1.4-2.6 TW. PRO - Pressure Retarded Osmosis, is a membrane-based method to produce salinity gradient power. In PRO, power generation is based on the osmotic water flow from the diluted to the concentrated side of the membrane, magnifying its flow and eventually discharging through a hydro-turbine to produce power.
In this work, a sensitivity analysis is conducted through full-scale modeling. The model contains the equations for velocities, pressures and concentrations for every point along the membrane module in both sides of the membrane, derived from considerations of mass and momentum conservation. These equations are solved numerically using a code written in MATLAB and APMonitor, a python-based differential-algebraic equation solver. Some of the parameters considered include different draw concentrations - seawater, SWRO-brine, and concentrated brine, as well as the membrane permeability for water and salt, and the membrane’s structure parameter. Importantly, the current study considered the impact of the type of spacer used, whose purpose is to keep the structural integrity of the feed and draw channels within the module. High-pressure operation on the draw side requires dense spacers resulting in a higher pressure loss. Another course of this work is a techno-economic investigation of PRO and RO-PRO on the basis of the parameters scope. That is an investigation of how large-scale constraints can affect the power production, PRO energy cost, and RO-PRO product water cost. Counter-current configuration, which consider more efficient because of a better conservation of the osmotic difference, and different hypothetic membrane characteristics were investigated as well. The results indicate that in by coupling PRO and RO, the specific energy consumption for desalination can be as low as the theoretic energy of separation of seawater from their salts, which is 1.1 kWh m-3, 50% of the common specific energy consumption (SEC) in a 50% recovery SWRO plant, with a small increase in the water cost compared to a 100% grid electricity supplied SWRO plant. With ‘dense’ spacers, the hydraulic loss found to be around 20 times larger than with ‘feed’ spacers. Small structure factor improved the efficiency due to reducing the concentration polarization between the bulk and the membrane. Together with a counter current configuration, it lowered the energy cost in 20%. The membranes part out of total cost found to be 70%, which makes membrane development crucial for PRO to become commercially viable. For that aim, also decisive policy measures should be taken, such as routing subsidies to renewables instead of fossil fuels.